PVC blade discussion

[Baling Wire]

relation to the plane of rotation for each station along the blade.  It seems as if the theory for pvc blades is to try and cut them to the shape that will give the proper engagement of the blade to the "phi" angle at the particular station.

The "Zubwoofer" design approach appears? to address the "cutting of them" so as to have this "proper engagement" of the relative wind.  This term "proper engagement" needs to be discussed and clarified in my mind too.  I believe the pipe can be cut to give the correct relationship; but, I do not believe that it is entirely a linear function?  Zubwoofer design works with 7 and 25 degree angles; and that the pipe diameter is not "too critical" and different blade lengths use the same angles??  From what I've determined computationaly? the 7 and 25 degree angles are pretty close.  These "phi" angles fit well for a tsr of 8; only thing the 25 degrees is NOT at the hub center.  The 25 degree cut should be at 1/8th "R"  Concerning the pipe diameter, one must have a large enough diameter so,  as these angles are cut, that there will be enough solidity to provide the necessary torque. I believe that the minimum pipe diameter for a 4 ft prop would be 8 inch?  Also, with a 7 degree tip, the chord there gets shorter as the pipe gets smaller and too small could lead to unwanted flexability.

A lot of pvc blades are mounted to the hub with bolts at or near the straight trailing edge.  I have read that curved/cambered airfoils yield Lift at "zero"

angle of attack.  In one of Hugh's books, he mentions that a 10% curvature i.e. 36 degrees, with an angle of attack of 3 degs. has a pretty good lift/drag ratio.

My question is: Using the correct size pipe, cutting it to the correct shape/degree so as to engage the proper "phi" for each station; now should ONE  A. adjust mounting on the trailing edge so as to introduce an angle of attack? or B.  introduce an angle of attack by varing the degrees of "curvature of cut" at each station by a fixed amount? or C.  put mounting bolts on tangent of trailing edge and have Zero angle of attack?  

Please feel free to correct any errors or add comments of your own.  I've made many templates and no blades.  I'd kind of like to get it right the first time or build a better mouse trap!!  Hi! Hi!    

BW

[Baling Wire]

After reviewing my notes, I noticed that I had made a mistake about the placement of the 25 degree segment width.  It is very near to 1/4 "R" for tsr 8 .  I also believe that a lot of pvc blades run in the 4 to 5.5 tsr range for one reason or another?  Am I wrong?

BW

[Flux]

I have never thought that bits of pvc pipe are worth analysing mathematically. At best they are best used for fairly low tsr and in small sizes.

I have not looked at any of the stuff that you mention so I am not sure what your Phi angle is. I presume it is the angle based on the line joining leading and trailing edge and you are comparing this to the setting angle ( pitch ) that Hugh gives for his blades.

The lift of a curved surface will be different from the flat faced aerofoil used by Hugh on the wooden blades.

7 deg at the tip combined with the curved surface I would have thought would give a tsr well below 8. I would expect you to be down to below tsr5 and those pvc things always seem to work best on slow machines. 25 deg near the root seems reasonable and it really doesn't matter much whether it is at the centre or 1/3 the way out. The inner third does nothing except help produce a bit more starting torque which is a good thing if you are using some draggy thing like a dc motor that needs a bit of starting.

I suspect that with small sizes of pipe the chord width is too small for the assumed tsr and increasing blade numbers would be a good thing.

The angle for zero lift varies significantly for different aerofoils, some have to be held well negative for zero lift so the setting angle would be different for various aerofoils.

I really don't think you will get reasonable results with pvc blades at tsr8, above tsr 5 you need to start using something that at least resembles an aerofoil. For tsr 4 then the pvc thing may work quite well if you get the correct angles and solidity. I doubt that you will get it with 2 blades if you use normal pipe sizes.

The only thing I did with pvc blades worked well enough with 6 blades and I must confess I made the shape closed to mk1 Marlec blades, with less twist than you are aiming for. Took longer to make than a decent set of wooden blades, I just can't see the point.

Flux